Method for cleaning waste water

ABSTRACT

Method for cleaning waste water, wherein the waste water (19) and oxygen gas (31) are flowing in opposite directions through a cylindrical multireactor (20) having a central rotatable pipe (24), the multireactor comprising a number of bioreactors and at least one separation chamber (75). The average pressure difference between two successive bioreactors is at most seven meter water column. The bioreactors are separated from each other by stationary walls (40, 41, 48, 55, 60), and each bioreactor has its own mixing device (50) for mixing waste water and oxygen gas.

The invention relates to a method for cleaning waste water, which makesuse of a device comprising at least one multireactor, which multireactorconsists of a cylindrical container, the axis of which is positionedvertically, and a rotatable central pipe coaxially positioned within thecontainer.

Such a device is generally known and is practically used on a largescale.

In order to improve the biological cleaning of the waste water, pureoxygen or an oxygen containing gas, hereafter called oxygen gas, issupplied. The problem is that the oxygen is bubbling up relatively fastthrough the waste water to be cleaned and only a small part of theoxygen gas supplied is used in the biological cleaning.

It is an object of the invention to provide a method as hereinbeforedefined wherein the oxygen gas is more effectively used.

According to the invention this object is achieved in that in eachmultireactor there are at least two bioreactors positioned above oneanother, and above said bioreactors there is at least one separationchamber, in that two adjacent bioreactors are separated from each otherby means of a horizontal mainly annular partition wall, the outer edgethereof being connected to the inner wall of the container, and theinner diameter being somewhat larger than the outer diameter of thepipe, and in that the supply of oxygen gas is provided in the lowestbioreactor of each multireactor.

Due to the presence of the annular partition walls the bubbling up ofthe oxygen gas is delayed considerably, whereby it can be used moreeffectively. Besides by a suitable choice of the form and position ofthe partition wall the upward directed movement of the oxygen gas can beused for mixing the oxygen gas and the waste water.

Other characteristics and advantages will become clear from thefollowing description, reference being made to the accompanyingdrawings, wherein

FIG. 1 is a schematic cross section of a first multireactor, as it canbe provided in a waste water cleaning system according to the invention,

FIG. 2 is a schematic cross section of a second multireactor, as it canbe provided in the cleaning system,

FIG. 3 is a schematic cross section of a third aultireactor, as it canbe applied in a device according to the invention, and

FIG. 4 is a diagram of a complete system according to the invention.

As represented in FIG. 1 the first multireactor comprises a cylindricalcontainer 20, comprising a cylinder wall 21, a bottom plate 22 and anupper wall 23. With its bottom plate 22 the container 20 is resting on afoundation, not shown, in such a way that the cylinder wall 21 ispositioned vertically. Coaxially with the cylinder wall 21 there isprovided a hollow pipe 24 in the container 20, the bottom end of saidpipe being rotatably supported by the bottom plate 22. This bottom endof the pipe 24 is constituted by a wall portion 25 having the form of atruncated cone cooperating with a conical part 26 connected to thebottom plate 22. A number of openings is provided in the wall portion25. The upper end of the pipe 24 is also constituted by a wall portion27 having the form of a truncated cone, which wall portion 27 is alsoprovided with openings. A stub shaft 28 is connected with the wallportion 27, and extends through an opening in the upper wall 23, andoutside the container 20 it is connected with an electromotor 29supported by the upper wall 23. The dimensions of the parts are chosenin such a way that the wall portion 25 is not touching the conical part26, but that the pipe 24 is mainly supported by the support of the motor29. Around the upper end of the pipe 24 a funnel shaped plate 35 isprovided, whereby the liquid contained therein is drained away throughthe openings in the wall portion 25. Waste water is added via pipe 19.

In the container 20 a number of horizontally positioned annularpartition plates is provided, which either are connected to the innerside of the cylinder wall 21 or to the pipe 24, thereby dividing thecontainer into a number of reaction spaces. In the lowest part of thecontainer 20 a supply system for oxygen gas is provided, which system inthe embodiment shown comprises a circularly formed pipe 30 provided witha number of openings and a supply pipe 31, one end of which is connectedto the circularly formed pipe 30, and the other end extending outsidethe container 20 and being connected to a system (not shown) forsupplying the oxygen gas.

As seen from the bottom upwards the first partition wall 40 in thecontainer 20 is formed by an annular plate 32 which is fixed to the pipe24 by means of a ring 33 with triangular cross section. A ring 34 withrectangular cross section is connected to the outer edge of plate 32,whereby a space with U-shaped cross section is formed under plate 32, inwhich space gas can be accumulated. Ring 34 has an outer diameter whichis smaller than the inner diameter of the cylinder wall 21, therebyforming an annular opening 36 between ring 34 and wall 41 through whichwaste water can flow. Preferably ring 34 comprises two concentric rings,which are interconnected by a number of blades at an oblique angle withrespect to the perpendicular direction, thereby forming a paddle wheel.

The next partition wall 41, as seen from the bottom upwards is formed byan annular plate 42, the outer edge of which is connected to the innerside of the cylinder wall 21 by means of a ring 43 with triangular crosssection. The inner edge of the plate 42 is formed by a conicaldownwardly sloping portion 44, which is further connected to an annularhorizontal portion 45. The inner diameter of portion 45 is larger thanthe outer diameter of pipe 44 thereby forming an annular opening 46through which waste water is allowed to flow. By this form of thepartition wall 41 a space is formed at its underside in which gas can beaccumulated.

Above the opening 44 there is a conical wall 48, its edge with thesmallest diameter being fixed to the pipe 24, whereas the lowerpositioned outer edge has a diameter which is slightly larger than theinner diameter of the portion 45. A number of openings is provided inwall 48 and an equal number of pipes 49 project through said openings.The parts of the pipes 49 extending above wall 48 have their ends all atthe same level above wall 48. The part of each pipe 49 extending underwall 48 is U-shaped bent and ends near the connection between wall 48and pipe 24. In this way siphon-like cut offs are formed, through whichthe gas accumulated under wall 48 can flow upwardly to the nextmultireactor.

Above the parts of the pipes 49 extending upwardly beyond the wall 48there is provided a diffusor 50 by which the gas bubbles coming from thepipes 48 are atomised into smaller bubbles. The diffusor 50 comprisestwo annular walls 52 and 53 which are connected by means of brackets 51.The walls 52 and 53 have a cross section in the shape of a part of acircle and are opposed with their convex sides thereby forming anarrowing-widening flowing section. The diffusor 50 is fixed to the pipe24.

A partition wall 55 situated above diffusor 50 has a construction whichis identical to the construction of the partition wall 40, an annularopening 56 being formed between the wall 55 and the cylinder wall 21.

Above the partition wall 55 there is a partition wall 60. The partitionwall 60 comprises a conical part 61, its construction being identical tothe wall 48 and itself being fixed to the pipe 24. To the outer edge ofpart 61 there is connected a horizontal annular plate 62.

To the inner and outer edge respectively of the plate 62 there isprovided a downwardly directed flange 63 and 64 respectively, flange 64being directed more downwardly than flange 63.

By a suitable choice of the diameters an annular opening 65 is formedbetween flange 64 and the cylinder wall 21. Under the opening 65 and tothe inner side of the wall 21 there is formed an annular projection 66having a triangular cross section. Hereby it is prevented that gasbubbles flow upwardly immediately through the opening 65. The gasbubbles are interrupted either in the space 70 under the conical part61, or in the space 71 formed by the plate 62 and the flanges 63 and 64.In the conical part 61 a number of siphon-like pipes 72 is providedcorresponding to pipes 49 in the wall 48.

Above the partition wall 60 there is provided a settling device 75. Thesettling device 75 comprises a number of equally spaced concentricplates 77 with a truncated conical cross section, the edge with thelarger diameter being at the underside. The under edges of these plates77 are fixed to a number of spokelike horizontal bars 76a which arefixed to the wall 21. The upper edges of the plates also are fixed to anumber of spokelike horizontal bars 76b which are also fixed to the wall21. To the inner ends of the spokes 76a and 76b an inner concentric ring78 is provided. The under edge of said ring 78 lies in the samehorizontal plane as the under edges of the rings 77 and the spokes 76a.The upper end of the ring 78 extends upwardly above the horizontal planeformed by the upper end of the rings 77 and the spokes 76b.

Above the settling device 75 there is a wall 80 corresponding to wall 48but having an outer diameter larger than the inner diameter of thetruncated conical wall formed by ring 78, wall 83 and ring 78 togetherforming siphon sealing. This means that the ring 78 extends into thespace 81 formed by wall 80, the pipe 24 and the horizontal planecorresponding to the under edge of the wall 80. The pipes 72 extendupwardly above the wall 80, and above the pipes 72 there is a diffusor82 corresponding to the diffusor 50.

Above the settling device 75 there is also provided a partition wall 83comprising an annular plate 84 which by means of a ring 85 withtriangular cross section is fixed to the inner side of the cylinder wall21. The partition wall 83 is positioned at a somewhat higher level thanthe under edge of the wall 80, and the inner diameter of the annularplate 84 is larger than the diameter of the wall 80 at that height,thereby forming an annular opening 86.

In the top portion of the container 20 there is provided a float 90. Thefloat 90 comprises two annular airtight containers 91 and 92 the uppersides of which are connected by means of an annular plate 93. In plate93 there is an opening through which a pipe 94 extends, which pipe alsoextends upwardly above the upper wall 23, and through which the oxygengas can be removed from the container 20. To the inner side of thecontainer 91 there is fixed a ring 95 with L-shaped cross section,forming a circumferential gutter. In this gutter there is formed asiphon 96. This siphon 96 comprises a horizontal plate 97 with the shapeof a segment of a circle which is fixed to the inner side of thecontainer 91 and a vertical plate 98 fixed to the straight edge of theplate 97 and extending downwardly from there into the ring 95. Into thespace defined by the plates 97 and 98 and the container 91 one end of apipe 99 is debouching, which via a flexible portion is extended to theoutside of the container.

The second multireactor shown in FIG. 2 consists of a cylindricalcontainer 120 comprising a cylinder wall 121, a bottom wall 122 and anupper wall 123. The container 120 rests with its bottom wall 122 on afoundation, not shown, in such a way that the cylinder wall 121 ispositioned vertically. Coaxially with the cylinder wall 121 a hollowpipe 124 is provided in the container 120, the bottom end of said pipebeing rotatably supported by the bottom plate 122. This bottom end ofthe pipe 124 is constituted by a wall portion 125 having the form of atruncated cone cooperating with a conical part 126 connected to thebottom plate 122. A number of openings is provided in the wall portion125. The upper end of the pipe 124 is also constituted by a wall portion127 having the form of a truncated cone, which wall portion 127 is alsoprovided with openings. A stub shaft 128 is connected with the wallportion 127 and extends through an opening in the upper wall, andoutside the container 120 it is connected with an electromotor 129supported by the upper wall 123.

In the container 120 a number of horizontally positioned annularpartition walls is provided, which either are connected to the innerside of the cylinder wall 121, or to the pipe 124, thereby dividing thecontainer into a number of reaction spaces. In the lowest part of thecontainer 120 a supply system for oxygen gas is provided which system inthe embodiment shown comprises a circularly formed pipe 130 providedwith a number of openings and a supply pipe 131, one end of which isconnected to the circularly formed pipe 130 and the other end extendingoutside the container 120 and being connected to a system (not shown)for supplying the oxygen gas.

As seen from the bottom upwards a first partition wall 140 is provided,the construction of which is identical to the construction of thepartition wall 40 described with respect to FIG. 1. Above the partitionwall 140 there is provided a partition wall 141 the construction ofwhich corresponds to the construction of the partition wall 41 describedwith respect to FIG. 1.

Above the partition wall 141 there is provided a conical wall 145 theconstruction of which is identical to the construction of wall 48described with respect to FIG. 1, and above this the constructioncomprising the partition wall 141 and the conical wall 145 is againrepeated by the partition wall 150 and the conical wall 155. Above theconical wall 155 there is provided a diffusor 160 the construction ofwhich corresponding to the construction of the diffusor 50 describedwith respect to FIG. 1, and above the diffusor 160 there is provided apartition wall corresponding to the partition wall 140.

Above the partition wall 165 there is provided a partition wall 170,comprising an annular plate 171 which is fixed to the inner side of thecylinder wall 121 by means of a ring 172 with triangular cross section.The inner edge of the plate 171 supports a ring 173 with U-shaped crosssection, the open end of which is directed upwardly and the edge of oneflange being connected to the inner edge of plate 171. The other flangeof the ring 173 extends into the space 174 defined by the pipe 124 and aring 175 with L-shaped cross section fixed thereto, said space 174having an open bottom end. In space 174 there is formed in this way asiphon sealing. On the upper side of the plate 171 there is fixed anumber of vertical plates 176, serving as braking plates for the liquidflow in this space and intended to convert said liquid flow into alaminated flow. Immediately under the ring 172 the wall 121 is providedwith an opening connected to a pipe 180 through which waste water can besupplied and oxygen gas can be removed. The pipe 180 is provided with aconnection 181 through which the waste water can be supplied to the pipe180.

Near the upper end of the container 120 and to the inner side of thecylinder wall 121 there is fixed the outer edge of a ring 185, slopingdownwardly in the direction of the center of the container 120. To theinner edge of the ring 185 a cylinder wall 186 is fixed extendingsomewhat above the plate 171. A ring 187 with L-shaped cross section isfixed to the outer side of the wall 186 thereby defining an upwardlyopen ended space 188. In the bottom of the space 188 there is an openingconnected to a pipe 189 extending outside the container 120 throughwhich waste water can be removed.

In the upper part of the container 120 a scraper 190 is fixed to thestub shaft 128, said scraper 190 comprising a number of radiallydirected blades 191. The scum collected by the scraper 190 can beremoved via a draining pipe 192 which is connected to the container 120just above the ring 185.

The third multireactor shown in FIG. 3 consists of a cylindricalcontainer 200 comprising a cylinder wall 201, a bottom wall 202 and anupper wall 203. The container 200 rests with its bottom wall 202 on afoundation, not shown, in such a way that the cylinder wall 201 ispositioned vertically. Coaxially with the cylinder wall 201 a hollowpipe 203 is provided in the container 200, the bottom end of said pipebeing fixed to the bottom plate 202 and the pipe extending to a defineddistance from the upper wall 203. The upper end of the pipe 203 is open,whereas near the bottom end a number of openings 204 is provided in thewall of the pipe 203. Above the openings 204 a ring 205 having the shapeof a truncated cone is fixed around the pipe 205. Said ring 205 iscooperating with a ring 216 having the shape of a truncated cone whichis fixed to the lower part of a pipe 206. The rings 205 and 216 togetherwith a space below a partition wall 222, which will be described lateron, define a siphon-like sealing between the container 201 and the spacein the pipe 206, said pipe 206 being coaxially with the pipe 203. Nearthe upper end of the pipe 203, the pipe 206 has a portion 207 withenlarged diameter, which portion is connected to the pipe 206 by meansof a conical wall portion 208, and to the stub shaft 210 by means of aconical wall portion 209. In each wall portion 208 and 209 a number ofopenings is provided.

Near the upper end of the pipe 203 and fixed to the circumferencethereof, a ring 211 with L-shaped cross section is provided, therebyforming an upwardly open gutter 212. The free flange of the ring 211extends into a space 213 defined by a ring 214 with L-shaped crosssection fixed to the inner side of the portion 207. The ring 214 definesa downwardly open space 213. The system consisting of the rings 211 and214 constitutes a siphon sealing between the spaces in pipes 203 and206. The stub shaft 210 extends through an opening in the upper plate203, and outside the container 200 it is connected to a motor 215supported by said upper plate 203.

In the lowest part of the container 200 a supply system for oxygen gasis provided, which system comprises a circularly formed pipe 220 and asupply pipe 221, and substantially corresponds to the system 30, 31described with respect to FIG. 1.

In the container 200 a number of horizontally positioned annularpartition plates is provided, which either are connected to the innerside of the cylinder wall 201, or to the pipe 206, thereby dividing thecontainer into a number of reaction spaces. As seen from the bottomupwards a first partition wall 222 is provided, the construction ofwhich being identical to the construction of the partition wall 40described with respect to FIG. 1. Above the wall 221 there is provided asystem consisting of a partition wall 225, a conical wall 226 and adiffusor 27, corresponding to the system 41, 48 and 50 as described withrespect to FIG. 1. Above this sytem a partition wall 228 is providedcorresponding to partition wall 222.

Above the partition wall 228 a partition wall 229 is provided,comprising an annular plate 230 which by means of a ring 231 withtriangular cross section is fixed to the inner side of the cylinder wall201. Fixed to the inner edge of the plate 230 is an edge of a conicalwall portion 232, which from plate 230 extends downwardly and inwardly.To the other edge of the wall portion 232 there is fixed a horizontallypositioned annular plate 233. Fixed to the inner edge of plate 233 isone edge of a conical wall portion 234, which from said edge is extendedupwardly and inwardly, the inner edge being positioned at a defineddistance from the circumference of the pipe 206.

Above the wall 229 and fixed to the pipe 206 there is provided a conicalwall 235 and a diffusor 236, corresponding substantially to the conicalwall 48 and the diffusor 50 as described with respect to FIG. 1. Thewall 235 is positioned in such a way and has such dimensions that thewall portion 234 extends into a triangular space defined by the wall 235and the pipe 206. Above the wall 229 there is also an annular pipe 237provided with openings for the supply of oxygen gas, which pipe 237 viaa pipe 238 is connected to an oxygen gas supply device provided outsidethe container 200.

Waste water from an upper region of the reaction space next underlyingthe uppermost chamber space of the container 200 will flow through theopenings provided in the conical wall portion 208 of pipe 206 and thencedownward in pipe 206 and through openings 206a therein into the reactionspace or chamber next underlying the partition wall 220. Thus, thecontainer 220 may be considered as comprising two multireactors, a firstone being positioned over the second and separated from it by, thoughhydrostatically connected with it through, the partition wall means at229, 234 and 235, with the waste water removing means of the firstmultireactor serving as the waste water supplying means of the secondmultireactor.

Above the partition wall 229 and fixed to the inner side of the cylinderwall there is a wall portion 240 comprising a plate in the shape of anearly half cone. One end of a pipe 241 is situated within the spacedefined by the wall portion 240, which pipe 241 is extended downwardlythrough an opening in the partition wall 229 until just above thepartition wall 225. By means of a T-joint a pipe 242 is connected to thepipe 241, which pipe 242 is extended through the cylinder wall 201 andconnected to the supply of oxygen gas. Above the wall portion 240 thereis an opening in the cylinder wall 201, which is connected to a pipe243, otherwise connected to the supply of waste water.

Above the diffusor 236 and fixed to the pipe 206 there is a partitionwall 244 corresponding substantially to the partition wall 40 describedwith respect to FIG. 1. Above this there is a partition wall 245 and awall 246 corresponding substantially to the partition wall 41 and thewall 48 as described with respect to FIG. 1. Further upwardly there isprovided a system comprising a partition wall 247, a settling device248, a wall 249, a partition wall 250 and a diffusor, correspondingsubstantially to the system comprising the partition wall 60, thesettling device 75, the wall 80, the diffusor 82 and the partition wall83 as described with respect to FIG. 1. Above the diffusor 251 there isprovided a partition wall 252 corresponding substantially to thepartition wall 40 as described with respect to FIG. 1.

Above the partition wall 252 there is provided a partition wall 253comprising an annular plate 254 which by means of a ring 255 withtriangular cross section is fixed to the inner side of the cylinder wall201. Fixed to the inner side of the plate 254 there is one edge of aflange of a ring 256 with substantially U-shaped cross section, formingan upwardly open gutter. The edge of the other flange of the ring 256extends into a downwardly open space defined by a ring 258 with L-shapedcross section fixed to the portion 207. By the rings 256 and 258 thereis defined a siphon-like sealing. Under the ring 255 there is an openingin the cylinder wall connected to a pipe 260 through which oxygen gascan be removed. A number of radially oriented plates 261 is fixed to thetop side of plate 254, in the same way as the plates 176 described withrespect to FIG. 2. A siphon-like connection 262 is provided on thecylinder wall 201, which connection 262 corresponds to the connection185 described with respect to FIG. 2. Waste water can be removedtherefrom via a pipe 263. A scraper 263, corresponding to the scraper190 of FIG. 2 is fixed to the stub shaft 210. Scraped scum can beremoved through pipe 265.

Hereafter the process realized by means of the above describedmultireactors will be described.

The known processes using activated mud for cleaning waste water arebased upon the formation of micro-organisms, which originate from thedecomposable biochemical compounds available in the waste water, wherebya new cellular material is generated which can easily be separated fromthe waste water to be cleaned.

The process as such is not homogeneous in that the waste water to becleaned contained qualitatively different substances, which aredecomposed by different kinds of micro-organisms each having their ownmetabolism. In the known processes use is made of the parameters takinginto account the average growth of the micro-organisms and the oxygenmanagement whereby the desired biological reactions can be performed.

The process comprises two phases:

The mixing, during simultaneously supplying oxygen, of the waste waterwith activated mud and the separation of the activated mud from thecleaned waste water.

The mixing of the activated mud and the waste water, duringsimultaneously supplying oxygen, is performed in different spaces eachhaving their own flowing characteristics.

The separation of the waste water and the floating micro-organisms isperformed either by settling or by flotation. This separation process isperformed in separation chambers, either in settling chambers asdescribed with respect to FIG. 1, or in flotation chambers as describedwith respect to FIG. 2.

Separation chambers and bioreactors can be integrated into one unit.Such a unit is indicated as a multireactor. In this way cleaning devicescan be more compact.

It is also possible to combine one multireactor with one or moreseparation chambers. Such a combination will be indicated as amultireactor combination.

In the known processes there is a relationship between the capacity ofthe separation chamber and the capacity of the bioreactor. Thisrelationship is dependent upon the concentration of activated mud in thewaste water during the cleaning process.

If the amount of activated mud per unit volume of the bioreactor isincreased, it is possible to decrease the overall capacity of thebioreactor. Otherwise this requires a longer rest time for the mixtureof waste water and micro-organisms in the settling chamber, in order tooptimize the separation between the sediment and the activated mud. At agiven capacity of the cleaning device it is therefore possible to reducethe dimensions of the device.

The mixing of the waste water with the oxygen and the activated mud inthe bioreactor is effected on the one side by using the kinetic energyof the oxygen gas supplied and on the other side by using a mechanicalstirring device.

The power required per unit of effective volume of the bioreactor is ameasure of the effectiveness of the mixing process, requiring a criticalvelocity of the liquid, at which velocity sedimentation is impossible.This effectiveness is a measure for the hydraulic characteristics of thebioreactor. In the known bioreactors the power required is more than 20W/m³.

The supply of oxygen containing gas or pure oxygen in the bioreactor isperformed by injecting the gas in the liquid via pipes or grids. Bymeans of the dimensions of the supply openings the dimensions of theformed gas bubbles can be controlled, whereby the effective contactsurface between the liquid and the gas can be increased, therebyimproving the absorption of oxygen by the liquid. Too small opening caneasily be obstructed whereby the flow resistance increases and pressureloss occurs.

The efficiency of the bioreactor is dependent on the extent of use ofthe oxygen supplied, and especially of the dissolving velocity of theoxygen in the waste water.

The dissolving velocity of the oxygen must be adapted to the oxygenconsumed by the activated mud. In this way the oxygen concentration canbe maintained at a level necessary to optimalize the metabolism of themicro-organisms, which metabolism defines the efficiency of the cleaningprocess.

If the activated mud consumes per unit of time more oxygen than isdissolved, there will be an oxygen deficiency, whereby the efficiency ofthe cleaning process decreases.

The cleaning velocity by means of oxygen, also called oxidationcapacity, is expressed by the quantity oxygen (gram) per unit of volume(m³) of liquid per unit of time (hour). It is a measure for the increaseof concentration of the micro-organisms, which are needed to create theconditions necessary to decompose great quantities of impurities perunit of volume.

The driving power for dissolving the oxygen in the liquid is theso-called oxygen deficiency, which can be defined as the differencebetween the oxygen concentration in a saturated solution and the oxygenconcentration available during the cleaning process.

The oxygen deficiency is the basic parameter for the velocity with whichthe oxygen is dissolved in the waste water and defines the grade of useof the oxygen supplied.

By the process according to the invention a capacity is achieved withrespect to the decomposition of either easily or not easily decomposablesubstances, the oxygen is dissolved with greater velocity and theconsumption of oxygen gas is greater than it was in known processes,whereas at the same time smaller devices can be used for cleaning thewaste water.

Essential for this process is that the dissolving of the oxygen in thewaste water and the mixing of the waste water with the activated mud isdone while the waste water is flowing through one or more multireactors,each multireactor comprising at least two bioreactors connected inseries, the hydrostatic pressure and the oxygen dissolving velocitybeing different in each bioreactor, and the pressure difference betweentwo successive bioreactors being not greater than 7 m water column.

The rest time of the waste water in each bioreactor is at least threeminutes, with the oxygen or the oxygen containing gas flowing as aresult of the upward pressure and moving through the liquid in thebioreactors in the direction of the bioreactor with the lowest pressure,and the output of the one reactor being connected with the supply of thenext reactor.

The gas is flowing through the above cited reactors from the bioreactorwith the highest pressure to the bioreactor with the lower pressure. Inthe upper portion of the bioreactor with the highest pressure the gas iscollected, whereupon it is supplied in the form of gas bubbles through asiphon-like pipe to the bioreactor with lower pressure. The dimensionsof the gas bubbles are such that these bubbles at a decompression of atmost four meter water column are decomposed into smaller gas bubbles.

The mixing of the content of the bioreactor and the even distribution ofthe gas bubbles in the bioreactors is performed by using the own energyof the gas supplied at the bottom of the bioreactors and the mechanicalenergy of the mixing device, the total energy supply being less than 20W/m³ of the capacity of the bioreactor (in favorable conditions even 15W/m³). In this case the mixing device 1s driven by one driving motorwhich is common for the whole multireactor.

Uncontrolled fluid flows between the fixed portions of the separatebioreactors, or of the separation chambers, and moving portions fixed tothe driving mechanisms of the bioreactor are prevented by a siphonsealing, using the gas which also serves as supply of oxygen to thebioreactors.

The separation from waste water free from sediment and leaving themultireactor and at the same time the concentration of activated mud to3% dry mass is effected in the flotation chamber, coupled to thehydrostatic system of the multireactor, in which the magnitude of thehydrostatic pressure in the multireactor is defined in accordance withthe level of the overflow gutter from which the mixture waste water,sediment and dissolved gas flows to the flotation chamber. The gas isused to transport the oxygen to the bioreactor having the highesthydrostatic pressure. The decompression of the liquid is defined by thepressure difference between the flotation chamber and the bioreactorfrom which the liquid is flowing upwardly. This pressure difference isat least seven meter water column.

A high concentration of activated mud in the bioreactors is obtained inthat the waste water is flowing through a series of alternatebioreactors and settling chambers which are hydrostatically connected.In the settling chambers the filtering capacity for the mixture wastewater and activated mud is choosen in such a way that the concentrationof activated mud in the retained mixture is kept at a constant level,which level is dependent upon the defined concentration limit of the mudin the bioreactor.

The mud separated in the settling chamber will be supplied to the nextlower bioreactor either by means of gravity or by means of a scrapingdevice. From this bioreactor the content of the settling chamber isreplenished.

The decrease of the amount of activated mud during the start up periodto a suitable value is performed by periodically drawing off cleanedwater from the bioreactor with the lowest pressure, no oxygen beingsupplied to the multireactor for at least 20 minutes. During this startup period the supply of waste water to the space with the highestpressure is maintained.

The selective development of micro-organisms is maintained bycontrolling the oxygen deficiency in, as seen in the flowing directionof the waste water, the first bioreactor, using distributors supplyingthe oxygen gas. In this way oxygen is supplied to two or morebioreactors, the oxygen supply for each bioreactor being independent ofthe oxygen supply to other bioreactors.

The reaching of the average equilibium of the pressure in successivebioreactors with a periodically changing supply of waste water can beeffected by a recirculation between the bioreactors. Such a systemsatisfies in an arrangement comprising three multireactors. In such anarrangement the multireactors are designated, as seen in the flowingdirection of the waste water as the multireactors of the first, secondand third degree respectively.

In FIG. 4 a complete system for cleaning waste water is schematicallyshown, comprising three multireactors connected in series andconstructed respectively according to FIG. 1, FIG. 2 and FIG. 3. At 350there is indicated an oxygen gas supply, whereas the supply of wastewater is indicated at 360.

From FIG. 4 the different flows are clear. Otherwise it will be obviousthat the system need not always be composed of three multireactors andthat, dependent on the pollution content, use can be made of onemultireactor, or of a combination of two or more multireactors, theirsequence being dependent on the circumstances.

It is also clear that the invention is not restricted to the embodimentas described and shown, but that within the scope of the claims a numberof modifications can be applied.

I claim:
 1. A process for microbiologically cleaning waste water whereinwaste water is treated with oxygen gas and mixed with activated mud,which comprises flowing the waste water through a plurality of mutuallysuperposed reactor chambers in succession at a rate such that the timeof retention of the waste water in each reactor chamber is at leastthree minutes, each said chamber being substantially separated from yetin hydrostatic communication with another in a vertically elongatecontainer; injecting oxygen gas into and mixing the gas in the form ofbubbles with the waste water in a lower of said chambers; in at leastone of said chambers collecting gas bubbles rising through the wastewater into at least one downwardly open horizontally extended pocketholding collected gas in contact with and under hydrostatic pressurefrom an underlying body of waste water and by said pressure forcingcollected gas from said pocket upwardly and into the waste water in anoverlying chamber of said container; removing gas from an upper chamberof said container; and in an uppermost chamber thereof separatingtreated waste water from activated mud for delivery of the treated waterfrom the container; and passing the activated mud downward into anunderlying chamber;said container being sufficiently tall and saidflowing of waste water being effected so that the waste water is passedwith a decompression of at least seven meters water column from thelowermost of said reactor chambers to said uppermost chamber.
 2. Aprocess according to claim 1, further comprising maintaining an averagehydrostatic pressure difference of less than four meters water columnbetween the respective bodies of waste water in successive reactorchambers.
 3. A process according to claim 1 or 2, said injecting intothe waste water in said lower reactor chamber being of oxygen gassupplied from outside said container; and into the waste water in eachhigher reaction chamber of the container injecting and mixing in theform of bubbles gas forced upwardly by hydrostatic pressure from saidpocket holding gas collected from and in contact with the waste water inthe next lower of said chambers.
 4. A process according to claim 3, saidinjecting of gas into the waste water in each higher reactor chamberbeing effected at least in part through at least one tube extendingupwardly from a U-shaped lower end portion thereof disposed in a saidpocket to an upper end thereof opening into the waste water in the nexthigher reactor chamber.
 5. A process according to claim 4, said mixingof gas into the waste water in each higher reactor chamber beingeffected least in part by subjecting the waste water therein directly atabove said upper end of said at least one tube to a rotating impelleraction so as to break up and distribute through the waste water bubblesof the gas delivered from said at least one tube.
 6. A process accordingto claim 1 or 2, and settling activated mud from the waste water in asettling chamber of said container disposed below and in hydrostaticcommunication with said uppermost chamber and passing the settled muddownward into and mixing it with the waste water in the next underlyingreactor chamber.
 7. A process according to claim 1 or 2, said injectinginto the waste water in said lower reactor chamber being of oxygen gassupplied from outside said container; into the waste water in eachhigher reactor chamber of the container injecting and mixing in the formof bubbles gas forced upwardly by hydrostatic pressure from a saidpocket holding gas collected from and in contact with the waste water inthe next lower of said chambers; and settling activated mud from thewaste water in a settling chamber of said container disposed below andin hydrostatic communication with said uppermost chamber and passing thesettled mud downward into and mixing it with the waste water in the nextunderlying reactor chamber.
 8. A process for microbiologically cleaningwaste water wherein waste water is treated with oxygen gas and mixedwith activated mud, which comprises flowing the waste water through aplurality of multireactors in succession;in each said multireactorflowing the waste water through a plurality of mutually superposed,hydrostatically communicating reactor chambers comprised in a verticallyelongate container and then into an uppermost chamber therein above andin hydrostatic communication with said reactor chambers, each saidcontainer being sufficiently tall and said flowing being effected sothat the time of retention of the waste water in each said reactorchamber is at least three minutes and so that the waste water is passedwith a decompression of at least seven meters water column from thelowermost of said chambers to said uppermost chambers; in at least onesaid reactor chamber of each multireactor injecting oxygen gas suppliedfrom outside the container into the waste water, mixing the gas in theform of bubbles with the waste water, collecting gas bubbles risingthough the waste water into at least one downwardly open, horizontallyextended pocket holding the collected gas in contact with and underhydrostatic pressure from underling waste water, and by said pressureforcing collected gas upwardly from a said pocket and bubbling it intothe waste water in an overlying chamber of the container; in saiduppermost chamber of each said container separating treated waste waterfrom solids for delivery of the treated water from the container;passing the treated waste water from one of said containers by gravityflow into and then through the next container as the waste water supplythereinto; and in at least one of said multireactors settling activatedmud from the waste water in a settling chamber disposed below and inhydrostatic communication with said uppermost chamber and passing thesettled mud downward into and mixing it with the waste water in the nextunderlying reactor chamber.
 9. A process according to claim 8, saidinjecting of oxygen gas supplied from outside the container beingeffected into the waste water in a lowermost reactor chamber of eachsaid multireactor, and into the waste water in each higher reactorchamber of the multireactor bubbling gas forced upwardly by hydrostaticpressure from a said pocket holding gas collected from and in contactwith the waste water in the next lower reactor chamber of the container.10. A process according to claim 8 or 9, said flowing of waste waterbeing effected continually with removal of treated waste water from thelast of said multireactors but with periodic interruption, for at leasttwenty minutes in each interruption period, of the oxygen gas supplyfrom outside said containers.